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I see some popular apps using subtle shadow effects in iOS apps. However, I don't see anything explicitly mentioned in the iOS guidelines (to use or not to use).
I want to use some minimal drop shadow effects for the cards. So, would like to know if there's any reference to substantiate my design.
No, it's not an anti-pattern. And yes, there is some justification for your design choice. The Apple HIG states:
Minimal use of bezels, gradients, and drop shadows keep the interface light and airy, while ensuring that content is paramount.
Note that Depth is one of Apple's key design principles. From the HIG:
Depth. Distinct visual layers and realistic motion convey hierarchy, impart vitality, and facilitate understanding. Touch and discoverability heighten delight and enable access to functionality and additional content without losing context. Transitions provide a sense of depth as you navigate through content.
Shadows can help you convey depth. Google says in their Material Design guidelines that it is the only visual indicator of depth in an interface.
Shadows provide important visual cues about objects’ depth and directional movement. They are the only visual cue indicating the amount of separation between surfaces. An object’s elevation determines the appearance of its shadow.
If you are designing cross-platform, you're going to want to make subtle use of shadows to convey depth because both Apple and Google, key design influencers, use depth as a key design principle. Google is explicit about it while Apple just wants you to keep shadows to a minimum.
This is something I'm testing now. The departure from flat design was prompted by this NNG article: https://www.nngroup.com/articles/flat-ui-less-attention-cause-uncertainty/.
So far, the buttons with a little shadow are performing a little better than the flat versions, but it hasn't reached statistical significance yet.
Cite from the iOS Human Interface Guidelines: "Minimal use of bezels, gradients, and drop shadows keep the interface light and airy, while ensuring that content is paramount."
https://developer.apple.com/ios/human-interface-guidelines/overview/themes/
